The present invention relates to a forge-proof document comprising a security feature in the form of a perforation pattern which displays gray tones when viewed against a bright background.
Such a document is known from W098/19869.
Although the prior art document in question provides a very good security against forgery, it is important to develop new security features in respect of the technical potential of forgers.
For this purpose, the present invention provides the measure that the document is manufactured from a material which transmits light to a limited extent, that the perforation extends over only a part of the thickness of the document at the position of the perforation, and that the thickness of the remaining part of the document at the position of the perforation is modulated in accordance with the image to be displayed.
This measure results in a further degree of difficulty; the determining factor for displaying the gray tone of the perforation, and therewith the image, is determined by the remaining thickness of the document. This means that the depth of the non-continuous perforation must be determined very precisely. The resulting thickness is after all the difference between two larger values, i.e., the thickness of the total document and the depth of the perforation.
According to another independent measure according to the invention, the perforation extends at an angle differing from 90xc2x0 relative to the main plane of the document. This has the result that the perforation cannot be arranged with very small drills, but that use will have to be made of a laser which, on the one hand, requires a large investment and, on the other, requires a high degree of technical knowledge.
This method of arranging provides the option of modulating the angle so as to obtain a gray-value modulation.
There is moreover the possibility, as in the classic straight perforations, of modulating the density of the perforation or the size, i.e., the diameter, thereof.
The perforation is preferably an image.
It is herein noted that the image as arranged by means of perforation can be subjected to a certain degree of image-processing. It is hereby possible to compensate the features of the image lost due to the necessary quantization. An example of such an image-processing is xe2x80x9ccontour enhancement.xe2x80x9d
The invention is also applicable to perforation patterns which do not represent an image, but which represent an alphanumeric expression or a code.
It will be apparent that a combination of these possibilities can be applied. Such an oblique perforation can of course be combined with a normal straight perforation This combination provides the option of introducing an extra pattern. The main image, which is modulated in order to display gray tones, is for instance arranged herein with a straight perforation, while an additional feature, for instance in the form of a logo or letters, is arranged obliquely. The choice of the angle or other properties of the oblique perforation can be chosen such that during normal observation of the pattern at an angle of about 90xc2x0 the normal image appears, and that during observation at another angle the second image in the form of a logo or a letter combination becomes visible.
Another example is the arranging of two images at the same position on the carrier, although at different angles such that each eye sees its own image, and a stereo image is thus observed.
It will be apparent that this can be varied in numerous ways.
It is attractive herein to make use of a method wherein the document to be protected is irradiated by a laser source from two positions. It is of course possible herein to make use of two laser sources, although it is simpler to first irradiate the document in a first position with a laser source at a first angle and to then place the document in a different position wherein it is irradiated by the same laser source at a different angle.
When the laser source is placed close to the document, it is also possible to arrange a perforation at an angle differing from 90xc2x0; this is caused by the cone or pyramid shape inside which the laser light beam must displace in order to arrange the perforation. A pattern then results which has an increasing angle as the distance to the center of the image increases.
According to another preferred embodiment of the invention, the cross-section of the perforation in its transverse plane is unequal to a circle. The use of a laser source provides the possibility of performing such a perforation when there is a correct control of the positions of the laser spot. It is, in any case, practically impossible to obtain this with mechanical means in view of the fineness of the required pattern.
According to another preferred embodiment, a code is concealed in the representation of the image. Use can be made herein of the teachings already applied in graphic techniques, according to which it is possible to arrange changes in an image which are not visible to the normal eye and which result after a specific processing in a code being displayed.
Conversely, an immediately visible coding can also be chosen. The code can be used, for instance, to identify the machine on which the relevant product was made. The relevant machine can thus be identified in the case of improper use of a machine.
According to yet another embodiment, an intermediate layer is arranged in the document, which layer is provided with an ink.
The use of laser provides the possibility of complete removal, i.e., burning, evaporating and so on, of the material from which the document is manufactured. Contamination of the relevant layers of the document will herein hardly occur. When such a document is processed with mechanical means, a degree of smearing will occur.
This smearing can be observed particularly well when the ink is formed by ink sensitive in UV light.
According to another embodiment, perforations arranged in a carrier in a pattern representing an image are filled with an ink which lights up under UV light. Such a pattern becomes visible if it is illuminated with a UV light source.
In another embodiment, the inner sides of the perforations of such a pattern are provided with a layer, for instance by vapor-deposition of a reflecting metal layer, resulting in an image which is visible when viewed. Selective application of a layer to the inner side of all perforations is possible by arranging a removable foil before the perforations are arranged and removing it after said layer has been applied.
In another embodiment, the starting point is a carrier which is built up of material layers of different colors. By modulating the depth, the perforation can be made to end in the desired layer and thereby make a desired color visible. An image in color can thus be realized.
The invention further provides the option of arranging the perforation in a protected element mounted on the carrier, such as an optically variable element such as a hologram or a kinegram. Such security features are not accessible to a forger, since they are only transacted between one manufacturer and one buyer. By furthermore providing such a security feature with a personalized perforation pattern, the forger is also deprived of the possibility of transferring such an element from one document to another.
When the image represented by the perforation pattern corresponds with another image arranged on the document, it is possible to have the images coincide. This provides the option of having both images coincide precisely. This has as advantages: the problems for the forger and counterfeiter increase, verification becomes even faster and simpler, and no extra surface area is required for the perforated image.